The applicant has already proposed an electric power steering device that enables performance of steering assist operation for a short period of time in the event of a breakdown of a battery (Japanese patent application No. 2007-52263).
The electric power steering device has an auxiliary power source connected in series with a battery. The electric power steering device has a detector for detecting an output from the battery, a charging/discharging circuit, and a control circuit. The charging/discharging circuit charges the auxiliary power source by the battery and selectively brings about a first output state for supplying the motor with electric power according to an output voltage of the battery and a second output state for supplying the motor with the electric power according to an output voltage acquired in a state that the auxiliary power source is connected in series with the battery. The control circuit selects the output state of the charging/discharging circuit in accordance with required steering assist force. The control circuit selects the second output state when a breakdown of the battery is detected on the basis of a detection result from the detector, thereby supplying the motor with electric power by the output voltage of the auxiliary power source.
When the breakdown of the battery is detected, the control circuit selects the second output state of the charging/discharging circuit, thereby controlling the auxiliary power source to supply electric power to the motor. Consequently, even if the battery has broken down, the auxiliary power source would generate required steering assist force for a short period of time, so that operation of the electric power steering device can be maintained.